The current industrial practice for methanol production is a two-step reaction. The first step is the cleaning and reforming of methane (from natural gas) to carbon monoxide and hydrogen. This is followed by a second step consisting of a reaction between carbon monoxide and hydrogen in the presence of a solid catalyst to form methanol. This process is energy-intensive and uneconomical for all but very large scale methanol plants. The present invention relates to a method and apparatus for producing oxygenated hydrocarbons (especially methanol). More specifically, the embodiments are for converting alkanes, such as methane, ethane, propane and butane, into alkyl oxygenates, and in particular, for direct oxidation (under partial oxidation conditions) of methane (e.g. from natural gas) to alcohols and additional oxygenated hydrocarbons.
Various other methods and apparatuses for converting methane into methanol are also known. It is known to carry out a vapor-phase reformation of methane into synthesis gas (mixture of carbon monoxide and hydrogen) with its subsequent catalytic conversion into methanol, as disclosed, for example, in Karavaev M. M., Leonov B. E., et al “Technology of Synthetic Methanol”, Moscow, “Chemistry” 1984, pages 72-125. However, in order to realize this process, highly sophisticated equipment is needed to satisfy the high requirements of gas purity. Large quantities of energy are also consumed to obtain the synthesis gas and for its purification, with a significant number of intermittent stages in the process. This complexity makes the process economically unviable for medium and small enterprises with the capacity less than 2,000 tons/day.
Russian Patent No. 2,162,460 includes a source of hydrocarbon-containing gas, a compressor and a heater for compression and heating of this gas, and an oxygen-containing gas with a compressor. It further includes successively arranged reactors with alternating mixing and reaction zones and means to supply the hydrocarbon-containing gas into a first mixing zone of the reactor and the oxygen-containing gas into each mixing zone, a recuperative heat exchanger for cooling of the reaction mixture, a cooler-condenser, a partial condenser for separation of waste gases and liquid products with a subsequent separation of methanol, a pipeline for supply of the waste gas into the initial hydrocarbon-containing gas, and a pipeline for supply of waste oxygen-containing products into the first mixing zone of the reactor. In this apparatus, however, fast withdrawal of heat from the highly exothermic oxidation reaction of the hydrocarbon-containing gas is not achievable because of the inherent limitations of the heat exchanger. This leads to the need for a reduction in the quantity of supplied oxygen-containing gas and, further, it reduces the degree of conversion of the hydrocarbon-containing gas. Moreover, even with the use of oxygen as an oxidizer, it is not possible to provide an efficient recirculation of the hydrocarbon-containing gas due to the rapid increase in the concentration of carbon oxides. A significant part of the supplied oxygen is wasted for oxidation of carbon monoxide into carbon dioxide, which additionally reduces the degree of conversion of the initial hydrocarbon-containing gas to useful products and provides a further overheating of the reaction mixture. As it is necessary to cool these components in the gas-liquid mixture to recover the liquid fraction followed by subsequent heating before returning to the reactor, much energy is wasted. Furthermore, these components are recompressed after each pass through the recycle compressor. Thus, the substantial elimination of non-hydrocarbon components provides an opportunity to reduce operational and capital expenditures.
A further method and apparatus for producing methanol is disclosed in the patent document RU 2,200,731, in which compressed heated hydrocarbon-containing gas and compressed oxygen-containing gas are introduced into mixing zones of successively arranged reactors, and the reaction is performed with a controlled heat pick-up by cooling of the reaction mixture with water condensate so that steam is obtained, and a degree of cooling of the reaction mixture is regulated by parameters of escaping steam, which is used in a liquid product rectification stage.
U.S. Pat. Nos. 7,179,843, 7,456,327, 7,578,981, 7,642,293 and 7,687,669, and Published US patent applications Nos. 20060204413, 20060223892, 200638283, 20070100005 and 20070166212, which are incorporated herein by reference, describe efficient and low-cost methods and apparatuses for the direct partial oxidation of methane to methanol without use of a catalyst. However, because recycle systems are suspect to accumulation of inert gases naturally occurring in the hydrocarbon-gas or in the oxygen containing gas, as well as product gases of the partial oxidation reaction, by selectively removing these fractions from the recycle system efficiencies can be significantly improved and oxygen consumption reduced.
It is observed that the methods and apparatuses can be used with minimal processing of on-shore gas and gas-condensate deposits, as well as integrated with any gas consumer, such as power plants, gas distributing and gas reducing stations, chemical production facilities, etc., or small methane producers (e.g. coal mines, oil production (flares), landfills, farms, etc.).
Accordingly, there exists a need for an improved method and apparatus to permit for safe conversion of hydrocarbon-containing gas to liquids on off-shore rigs, and also for improved efficiencies and simplification to enable a closed cycle system for producing methanol and other oxygenated hydrocarbons.